A while back I remarked that the Babylon 5 spinoff Crusade died in part because the Sci-Fi channel had already committed to Farscape.
I was thinking about VR.5 recently. Had it survived into a second or third season, Anthony Stewart Head might not have been available for Buffy the Vampire Slayer. Can you imagine anyone else as Giles? Of course, given the nature of the show (they killed off a regular about five episodes in, and pulled an “everything you know is wrong” moment near the end of the season), it’s entirely possible that Oliver wouldn’t have made it through season 2 anyway, and he still would have gotten the role as Giles.